The optical spectrum analyzer (OSA) built-in to the optical network node usually has the capability of monitoring the parameters, such as, optical power, optical wavelength, optical path loss, optical signal to noise ratio (OSNR), and so on. The currently available techniques usually use digital communication analyzer (DCA) to observe the eye diagram of the optical signal to analyze the related parameters of the optical signal, such as, the ER value of the optical signal. The eye diagram is reconstructed by sequential sampling in the pseudorandom bit sequence (PRBS).
FIG. 1 shows an exemplary flowchart of a conventional method for computing ER value of the optical signal. As shown in FIG. 1, received optical signal 101 is converted to an electric pulse signal 110a through an optical-to-electrical conversion 110, which passes sequential sampling and eye diagram reconstruction 120 to become corresponding eye diagram 120a. Finally, eye diagram 120a is analyzed to obtain two optical powers 130 corresponding to level one and level zero of optical signal, and to obtain ER value.
Christopher M. Miller et. al. disclosed, in 1994, the use of eye diagram analyzer to execute the configuration of optical signal related parameters for digital transmission system, where the optical signal ER value is computed as:
      Extinction    ⁢                  ⁢    ratio    ⁢                  ⁢          (      dB      )        =      10    ·                  log        10            ⁡              [                              One            ⁢                                                  ⁢            level                                Zero            ⁢                                                  ⁢            level                          ]            The locations of the two average optical powers P1 and P0 corresponding to bit one and bit zero of optical signal are determined by the peak value of the histogram, and the locations of the peak values are the locations of level one and level zero.
U.S. Pat. No. 5,535,038 disclosed an apparatus and method for determining the ER by direct measurement of power. As shown in FIG. 2, means for measuring power 210 is used to measure the power Pmod of modulated signal. Take voltmeter (equivalent to measuring optical power) 220 as example to measure the average optical power Pav. The following equation is used to determine ER value:
      E    ⁢                  ⁢    R    =                    P        av            +              P        mod                            P        av            -              P        mod            The apparatus needs optical-to-electrical conversion 230 to convert the optical signal to electric signal.
U.S. Patent Publication No. 2007/0109533 disclosed a non-sampling-based Q-factor measuring apparatus and method. As shown in FIG. 3, this technique uses an optical signal domain conversion module to convert the optical signal power waveform to be monitored into signal format of other domains, such as, using a power-to-wavelength conversion module 310 to convert into the optical wavelength waveform. When the optical signal is converted from the optical power waveform into optical wavelength waveform, the Q-factor of the optical signal in the optical wavelength domain is defined as:
  Q  ≡                              μ          1                ⁡                  (          nm          )                    -                        μ          0                ⁡                  (          nm          )                                              σ          1                ⁡                  (          nm          )                    +                        σ          0                ⁡                  (          nm          )                    where μ1(nm), μ0(nm) are the average wavelengths of the level one and level zero of optical signal respectively, and σ1(nm) and σ0(nm) are the wavelength standard deviations of the level one and level zero of optical signal respectively.
The aforementioned techniques must collaborate with the optical filter to process wavelength division multiplexing (WDM) multichannel optical signal.
As shown in FIG. 4, in time domain, after the optical signal is transmitted by optical fiber, the output optical power waveform 410 (the solid line) and the chirp waveform 420 (the dash line) of the single wavelength directly modulated laser (DML) show high degree of resemblance. Optical signal level one and level zero have different optical frequency offset, that is, the chirp effect. Therefore, it is necessary to use an optical spectrum analyzing unit, such as, optical spectrum analyzer, to monitor the ER value of optical signals in the optical domain.